The invention is related generally to drive mechanisms for medical infusion pumps, and more particularly, to a system and method for retaining the plunger of a fluid container at the drive mechanism.
The infusion of fluids such as parenteral fluids into the human body is accomplished in many cases by means of a syringe pump having a lead screw and a screw drive mechanism which translates the rotational motion of the lead screw into linear motion. A syringe plunger driver is connected to the screw drive mechanism for connecting the linear motion of the screw drive mechanism to the syringe plunger to empty the syringe.
Because syringes are of different sizes and are filled to different levels with infusion fluids, the extension of the plunger from the syringe barrel will differ from syringe to syringe. Many screw drive mechanisms include a disengagement mechanism that the operator uses to disengage the lead screw drive mechanism from the lead screw threads. Once disengaged, the operator may move the plunger driver along the lead screw to the position of the extended syringe plunger, and then engage both the syringe plunger with the plunger driver and the lead screw drive mechanism with the threads of the lead screw at the new position. It is desirable that this disengagement mechanism and this plunger driver mechanism be easy to use to facilitate operator usage of the pump.
As is well known, syringes vary in size among manufacturers. Even syringes designed to contain the same quantity of fluid can vary substantially in outer dimensions of both length and diameter from manufacturer to manufacturer. In some prior pumps, only a single syringe from a single manufacturer could be used, thereby greatly restricting the usefulness of the pump. When that particular syringe type was not available, the pump could not be used. Pumps were also developed that could receive different sized syringes; however, obstacles still exist. Not only can syringe barrel diameters vary from syringe to syringe, but also the length of the syringe barrel, the plunger length, the plunger flange diameter, and thickness of the plunger flange can all vary. All of these physical variables would have to be considered when designing a plunger driver system for a syringe pump if that pump is to be capable of handling syringes of differing capacities from a variety of manufacturers.
In a syringe pump plunger driver, the driver typically engages the plunger flange and holds that flange in a fixed relationship with a pushing surface which is part of the plunger driver. The pushing surface contacts the plunger flange and applies force to move the plunger flange into the syringe to expel the syringe contents. Some plunger drivers include a retainer structure that operates as an anti-siphon feature to prevent the plunger from moving into the barrel and emptying the syringe at a rate in excess of the programmed movement rate of the pushing surface when under a negative pressure condition downstream. It is desirable to avoid a siphoning condition as the rate of administration of a fluid from the syringe is typically prescribed for a patient and exceeding that rate may not meet the requirements of the prescription. This is particularly true in the case where the medicament is to be administered to the patient at a very low flow rate. Even a small amount of siphoning can exceed the prescribed rate.
In the case where a syringe pump is to accommodate a wide variety of different syringe sizes, a plunger driver system is needed which will precisely hold each syringe in correct alignment with the pushing surface of the plunger driver. The system must also assure that the plunger will be firmly held in the plunger driver and that the plunger detector system will detect the presence and absence of a syringe and provide an alarm in the case of a dislodged or absent plunger. Additionally, it would be desirable if such a system were easy to use.
In the case of one syringe pump driver system, easy-to-use rotating arms exist that capture the plunger flange between themselves as well as capture the plunger flange at the driver. While this arrangement provided a substantial improvement in the art, the rotating arms are fixed in longitudinal position in relation to the driver and are located so that the driver system can accept a wide variety of syringes. Therefore, the longitudinal position of the rotating arms is selected to accommodate the thickest syringe plunger flange. Consequently, for thinner plunge flanges, some longitudinal movement between the driver and the arms can occur.
In another driver system, the syringe plunger flange is retained well but due to the configuration of the driver system, loading the syringe is often awkward. In one design known as the xe2x80x9csprung plate,xe2x80x9d a plate is mounted to the plunger drive head and is intended to trap the syringe plunger flange between itself and the pushing surface of the plunger drive. However, loading the syringe properly for this design is not intuitive. In most cases, operators attempt to load the syringe barrel first and then try to load the plunger. Difficulty is encountered when trying to load the plunger in this sequence as the plunger stem must now be forced outwards in order to locate the plunger flange behind the sprung plate. This in turn requires some movement of the syringe barrel; however, there is a barrel clamp over the syringe barrel which has a bias device trying to prevent this very movement.
Hence, those skilled in the art have recognized a need for a plunger driver system and method that are capable of handling different sizes of syringes while still presenting a relatively easy system to use to the operator of the pump. Such a system and method should provide a mechanism to align, firmly engage, and detect the presence of the plunger of each of the syringes specified for the pump. Further, such a system and method should be capable of resisting siphoning of the syringe contents from syringes of all sizes usable in the pump. The invention fulfills these needs and others.
The present invention is directed to a system and method for driving a syringe plunger while resisting siphoning. At the same time, the system and method in accordance with the invention permit the use of syringes of different sizes as well as facilitating the loading of a syringe.
In a first aspect in accordance with the invention, a syringe plunger driver system is provided for engaging syringe plungers of different sizes, each plunger having a plunger piston, a plunger flange, and a plunger stem interconnecting the piston with the flange and each plunger forming a part of a syringe, each syringe having a barrel into and out of which the plunger moves, each plunger flange having an inner side facing the syringe barrel and an outer side, the plunger driver system having a drive head adapted to move the syringe plunger into the syringe barrel in an operation mode, the driver system comprises a pushing surface located on the drive head adapted to press against the outer side of the plunger flange to move the flange toward the barrel during the operation mode, a plunger retainer located on the drive head adapted to engage the inner side of the plunger flange and retain the flange in contact with the pushing surface, the plunger retainer also adapted to adjust itself to the size of the plunger, and a first bias device connected with the plunger retainer to bias the plunger retainer towards the pushing surface, whereby siphoning is resisted.
In a further aspect, the driver system further comprises an activating lever interconnected with the plunger retainer and having a first position at which the lever moves the plunger retainer into a syringe plunger non-engagement position to permit easy loading of the syringe plunger in the driver system. In more detailed aspects, the activating lever in its first position moves the plunger retainer outward and forward into the syringe plunger non-engagement position in opposition to the first bias device. The first bias device comprises a spring connected with the plunger retainer that biases the retainer towards the pushing surface, the spring having enough force to retain a plunger flange positioned between the pushing surface and the retainer in contact with the pushing surface during the operation mode.
In other aspects in accordance with the invention, the plunger retainer comprises a first pivotally mounted arm and the first bias device is adapted to bias the first arm towards the pushing surface. The retainer further comprises a second bias device adapted to bias the first arm pivotally inward toward a plunger stem mounted in the syringe driver system. In more detailed aspects, the retainer further comprises a second pivotally mounted arm biased toward the pushing surface. A third biasing device is adapted to bias the second arm pivotally inward toward a plunger stem mounted in the syringe driver system. An activating lever is interconnected with the first and second arms and has a first position at which the lever pivotally moves the first and second arms outward and forward into a syringe plunger non-engagement position in opposition to the biasing force on the first and second arms whereby easy loading of a syringe plunger is facilitated. The lever is interconnected to the first and second arms such that when the lever is moved to its first position, the lever causes the arms to first move outward and then to move forward. Further, the arms are mounted to the drive head such that the arms adjust themselves to the size of the plunger mounted to the pushing surface. The lever has a second position at which the lever does not apply force opposing the biasing devices on the first and second arms so that the arms may move toward each other and toward the pushing surface to capture a syringe plunger. Further, the lever is interconnected to the first and second arms such that when the lever is moved to its second position, the lever causes the arms to first move inward toward each other and then to move toward the pushing surface.
Other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, illustrating by way of example the features of the invention.